The invention relates to an X-ray CT examination installation, with an X-ray tube with a focus, which creates a fan beam or a conical beam which X-rays the whole of a detector at a fixed distance from the focus, and with an examination carriage, for recording an object to be examined, which has an axis of rotation rotatable perpendicular to the fan beam. The invention also relates to a CT method of examining objects, in particular of various size, by means of an above-named X-ray CT examination installation.
There are currently two examination methods in industrial computed tomography (CT). One is a translation/rotation tomography and the other a rotation tomography. In both cases a fan beam which X-rays the whole of a one-dimensional detector is masked before a focal point, the focus, of an X-ray source. Both the X-ray source and also the detector are fixed. An object to be examined, which is rotated about an axis perpendicular to the plane of the fan, is inserted between them into the fan beam in order that the object can be reconstructed. The distance between X-ray tube and detector can be altered, likewise the position of the object which is arranged on a turntable so that the geometric enlargement can be matched to the requirements in each case. The individual horizontal layers of the object are recorded by progressively changing the height of the object or the X-ray tube and the detector. Instead of using a fan beam it is possible to use a conical beam and to project this onto a two-dimensional detector. A layer-by-layer scanning can then be dispensed with, depending on the size of the object.
In rotation tomography a complete measured data record is thus created because the whole of the object to be examined lies in the section plane in the fan beam and projections are recorded from at least 180° plus aperture angle of the fan beam. This method is fast, but the size of the beam fan determines the maximum size of the object which can be tomographed in this arrangement. This size is also called measuring circle. The relationship also applies in reverse, i.e. in order that a larger object can be tomographed, the measuring circle must be larger, i.e. a fan beam with larger aperture angle and thus also a larger detector are used. With this method even comparatively small objects require large installations with a long detector and a large distance between focus and detector. Technical limitations result from the maximum angle of radiation of the ray source which limits the aperture angle, and the size of the detector.
If the fan beam does not cover the whole object, this fan beam can be artificially widened by moving either the detector or the object sideways. This is translation/rotation tomography. However, there must be alternating linear and rotary movements, which is time-consuming and also requires for the transverse movement a linear axis which, over the whole distance covered, must ensure with a high degree of accuracy the right-angled alignment of the axis of rotation to the plane of the fan beam.
If, instead of a two-dimensional fan beam, a three-dimensional conical beam is used, the available examination volume is confined between the focus of the X-ray source and the corners of the mostly square two-dimensional surface detector or the edge contour of an inserted image-recording device. Otherwise the above statements apply by analogy.
Given a typical size and quantity distribution of objects to be examined—there are frequently many small and few large objects—it has hitherto been necessary to design an examination installation such that the large objects can be examined in all cases. Disadvantages result from this with regard to the size of the whole examination installation and with regard to the achievable measuring time.